The cold winter months, with their low temperatures and long nights, can prove challenging for many creatures. Food may be hard to come by and energetic costs high, so many insects, birds and mammals adopt different strategies to see out the winter days and wait for the arrival of spring.
Some birds, including many of our familiar summer songsters, will have migrated south to seek out more favourable conditions. It is those species that feed on insects and other invertebrates that would face the greatest difficulties were they to remain here during the winter and so many move into Africa, perhaps crossing the Equator to take advantage of the bounty that follows the seasonal rains. Some insect-eating species, however, choose to remain here. Wrens, for example, manage to scrape a meagre living by maintaining winter territories, often establishing these in river or lakeside habitats, where the damp conditions favour higher levels of insect activity. Others, such as the Pied Wagtail, seek the warmth of commercial glasshouses or the waste heat of city centres to reduce their energetic costs overnight.
Some of our insects over-winter as adults, perhaps entering torpor, reducing their ‘running costs’ and lowering their energetic demands. A few of our resident butterflies, for example, overwinter as adults, while nine species overwinter as eggs and eleven as pupae; the majority, however, spend the winter as caterpillars. It is also interesting to note that, with the exception of the speckled wood butterfly, the hibernating phase is always the same in a given butterfly species. Only in the speckled wood can hibernation occur as either a caterpillar or as a pupa. Other insects have a life cycle that sees eggs laid in summer or autumn used to secure passage through into spring. Eggs can be deposited in sheltered locations, they are often robust and require no external nutrition.
Mammals tend to cope with the conditions rather well and many species remain active throughout the winter, some even using these months for their mating season. A small number of mammal species enter hibernation, using fat reserves laid down during the bountiful conditions of autumn to get them through the winter. Others reduce the amount of time that they are active or retire to more favourable habitats.
What is particularly interesting about all this is the way in which so many different strategies are adopted. What suits one species does not necessarily suit another and even closely related species may do something completely different. This highlights that there are different evolutionary solutions to a common problem and this is one of the reasons why the study of natural history is so engaging. There is always something new to see and to discover.